MTH_251_intorduction_to_the_first_derivative
Functions, Derivatives, and Antiderivatives
Activity 21
Functions, derivatives, and antiderivatives have many entangled properties. For example, over intervals where the first derivative of a function is always positive, we know that the function itself is always increasing. (Do you understand why?)
Many of these relationships can be expressed graphically. Consequently, it is imperative that you fully understand the meaning of some commonly used graphical expressions. These expressions are loosely defined in Table 21.1.
Table 21.1: Some Common Graphical Phrases
|The function is positive |The function is negative |
|This means that the vertical-coordinate of the point on the function |This means that the vertical-coordinate of the point on the function |
|is positive. As such, a function is positive whenever it lies above |is negative. As such, a function is negative whenever it lies below |
|the horizontal axis. |the horizontal axis. |
|The function is increasing |The function is decreasing |
|This means that the vertical-coordinate of the function consistently |This means that the vertical-coordinate of the function consistently |
|increases as you move along the curve from left to right. Linear |decreases as you move along the curve from left to right. Linear |
|functions with positive slope are always increasing. |functions with negative slope are always decreasing. |
|The function is concave up |The function is concave down |
|A function is concave up at a if the tangent line to the function at a|A function is concave down at a if the tangent line to the function at|
|lies below the curve. An upward opening parabola is everywhere |a lies above the curve. A downward opening parabola is everywhere |
|concave up. |concave down. |
Problem 21.1
Answer each of the following questions in reference to the function shown in Figure 21.1. Each answer is an interval (or intervals) along the x-axis. Use interval notation when expressing your answers. Make each interval as wide as possible; that is, do not break an interval into pieces if the interval does not need to be broken up.
21.1.1 Over what intervals is the function positive?
21.1.2 Over what intervals is the function negative?
21.1.3 Over what intervals is the function increasing?
21.1.4 Over what intervals is the function decreasing?
21.1.5 Over what intervals is the function concave up?
21.1.6 Over what intervals is the function concave down?
21.1.7 Over what intervals is the function linear?
21.1.8 Over what intervals is the function constant?
Activity 22
Much information about a function’s first derivative can be gleaned simply by looking at a graph of the function. In fact, a person with good visual skills can “see” the graph of the derivative while looking at the graph of the function. This activity focuses on helping you develop that skill.
Problem 22.1
A parabolic function is shown in Figure 22.1. Each question in this problem is in reference to that function.
22.1.1 Several values of the function [pic] are given in Table 22.1. For each given value draw a nice long line segment at the corresponding point on g whose slope is equal to the value of [pic]. If we think of these line segments as actual lines, what do we call the lines?
22.1.2 What is the value of [pic] at 1? How do you know? Go ahead and enter that value into Table 22.1.
22.1.3 The function g is symmetric across the line [pic]; that is, if we move equal distance to the left and right from this line the corresponding y-coordinates on g are always equal. Notice that the slopes of the tangent lines are “equal but opposite” at points that are equally removed from the axis of symmetry; this is reflected in the values of [pic] and [pic]. Use the idea of “equal but opposite slope equidistance from the axis of symmetry” to complete Table 22.1.
22.1.4 Plot the points from Table 22.1 onto Figure 22.2 and connect the dots. Determine the formula for the resultant linear function.
22.1.5 The formula for g is [pic]. Use Definition 19.1 to determine the formula for [pic].
22.1.6 The line you drew onto Figure 22.2 is not a tangent line to g. Just what exactly is this line?
Problem 22.2
A function f is shown in Figure 22.3 and the corresponding first derivative function [pic] is shown in Figure 22.4. Answer each of the following questions referencing these two functions.
22.2.1 Draw the tangent line to f at the three points indicated in Figure 22.3 after first using the graph of [pic] to determine the exact slope of the respective tangent lines.
22.2.2 Write a sentence relating the slope of the tangent line to f with the corresponding y-coordinate on [pic].
22.2.3 Copy each of the following phrases onto your paper and supply the words or phrases that correctly complete each sentence.
• Over the interval where [pic] is always negative f is always .
• Over the intervals where [pic] is always positive f is always .
• Over the interval where [pic] is always increasing f is always .
• Over the interval where [pic] is always decreasing f is always .
Problem 22.3
In each of figures 22.5 and 22.6 a function (the thin curve) is given; both of these functions are symmetric about the y-axis. The first derivative of each function (the thick curves) have been drawn over the interval [pic]. Use the given portion of the first derivative together with the symmetry of the function to help you draw each first derivative over the interval [pic].
Problem 22.4
A graph of the function [pic] is shown in Figure 22.7; call this function f.
22.4.1 Except at 0, there is something that is always true about the value of [pic]; what is the common trait?
22.4.2 Use Definition 19.1 to find the formula for [pic].
22.4.3 Does the formula for [pic] support your answer to problem 22.4.1?
22.4.4 Use the formula for [pic] to determine the horizontal and vertical asymptotes for the graph of [pic].
22.4.5 Keeping it simple, draw onto Figure 22.8 a curve with the asymptotes found in problem 22.4.4 and the property determined in problem 22.4.1. Does the curve you drew have the properties you would expect to see in the first derivative of f ? For example, f is concave down over [pic] and concave up over [pic]; what are the corresponding differences in the behavior of [pic] over those two intervals?
Problem 22.5
A graph of the function [pic] is shown in Figure 22.9. The absolute minimum value ever obtained by [pic] is [pic]. With that in mind, draw [pic] onto Figure 22.10. Make sure that you draw and label any and all necessary asymptotes. Make sure that your graph of [pic] adequately reflects the symmetry in the graph of [pic].
Problem 22.6
A function, w, is shown in Figure 22.11. A larger version of Figure 22.11 is available in Appendix B (page B3). Answer each of the following questions in reference to this function.
22.6.1 An inflection point is a point where the concavity of a function changes. The inflection points on w occur at 2, 3.25, and 6. With that in mind, over each interval stated in Table 22.3 exactly two of the words in Table 22.2 apply to[pic]. Complete Table 22.2 with the appropriate pairs of words.
22.6.2 In Table 22.4, three possible values are given for [pic] at several values of x. In each case, one of the values is correct. Use tangent lines to w to determine each of the correct values. (This is where you probably want to use the graph on page B3.)
22.6.3 The value of [pic] is the same at 2, 4, and 7. What is this common value?
22.6.4 Put it all together and draw [pic] onto Figure 22.12.
Activity 23
A function is said to be nondifferentiable at any value its first derivative is undefined. There are three graphical behaviors that lead to non-differentiability.
• f is nondifferentiable at a if f is discontinuous at a.
• f is nondifferentiable at a if the slope of f is different from the left and right at a.
• f is nondifferentiable at a if f has a vertical tangent line at a.
Problem 23.1
Consider the function k shown in Figure 23.1.
23.1.1 There are four values where k is nondifferentiable; what are these values?
23.1.2 Draw [pic]onto Figure 23.2.
Problem 23.2
Consider the function g shown in Figure 23.3.
23.2.1 [pic]has been drawn onto Figure 23.4 over the interval [pic]. Use the piece-wise symmetry and periodic behavior of g to help you draw the remainder of [pic] over [pic]
23.2.2 What power math word applies to g at −5, 0, and 5?
23.2.3 What two power math words apply to [pic] at −5, 0, and 5?
Activity 24
Seeing as the first derivative of f is a function in its own right, [pic] must have its own first derivative. The first derivative of [pic] is the second derivative of f and is symbolized as [pic] (f double-prime). Likewise, [pic](f triple-prime) is the first derivative of [pic], the second derivative of [pic], and the third derivative of f.
All of the graphical relationships you’ve established between f and [pic] work their way down the derivative chain; this is illustrated in tables 24.1, 24.2, and 24.3.
Problem 24.1
Extrapolating from tables 24.1 and 24.2, what must be true about [pic] over intervals where [pic] is, respectively, positive, negative, and constantly zero?
Problem 24.2
A function, g, and its first three derivatives are shown in figures 24.1-24.4, although not in that order. Determine which curve is which function ([pic], [pic], [pic], and [pic]).
Problem 24.3
Three containers are shown in figures 24.5-24.7. Each of the following questions are in reference to these containers.
24.3.1 Suppose that water is being poured into each of the containers at a constant rate. Let [pic], [pic], and [pic] be the heights (measured in cm) of the liquid in containers 24.5-24.7, respectively, t seconds after the water began to fill the containers. What would you expect the sign to be on the second derivative functions [pic], [pic], [pic] while the containers are being filled? (Hint: Think about the shape of the curves [pic], [pic], and [pic].)
24.3.2 Suppose that water is being drained from each of the containers at a constant rate. Let [pic], [pic], and [pic] be the heights (measured in cm) of the liquid remaining in the containers t seconds after the water began to drain. What would you expect the sign to be on the second derivative functions [pic], [pic], [pic] while the containers are being drained?
Problem 24.4
During the recession of 2008-2009, the total number of employed Americans decreased every month. One month a talking head on the television made the observation that “at least the second derivative was positive this month.” Why was it a good thing that the second derivative was positive?
Problem 24.5
During the early 1980s the problem was inflation. Every month the average price for a gallon of milk was higher than the month before. Was it a good thing when the second derivative of this function was positive? Explain.
Activity 25
The derivative continuum can be expressed backwards as well as forwards. When you move from function to function in the reverse direction the resultant functions are called antiderivatives and the process is called antidifferentiation. These relationships are shown in figures 25.1 and 25.2.
[pic]
Figure 25.1: Differentiating
[pic]
Figure 25.2: Antidifferentiating
There are (at least) two important differences between the differentiation chain and the antidifferentiation chain (besides their reversed order).
• When you differentiate, the resultant function is unique. When you antidifferentiate, you do not get a unique function - you get a family of functions; specifically, you get a set of parallel curves.
• We introduce a new function in the antidifferentiation chain. We say that F is an antiderivative of f. This is where we stop in that direction; we do not have a variable name for an antiderivative of F.
Since F is considered an antiderivative of f, it must be the case that f is the first derivative of F. Hence we can add F to our derivative chain resulting in Figure 25.3
[pic]
Figure 25.3: Differentiating
Problem 25.1
Each of the linear functions in Figure 25.4 have the same first derivative function.
25.1.1 Draw this common first derivative function onto Figure 25.4 and label it g.
25.1.2 Each of the given lines Figure 25.4 is called what in relation to g?
Problem 25.2
The function f is shown in Figure 25.5. Reference this function in the following questions.
25.2.1 At what values of x is f nondifferentiable?
25.2.2 At what values of x are antiderivatives of f nondifferentiable?
25.2.3 Draw onto Figure 25.6 the continuous antiderivative of f that passes through the point [pic]. Please note that every antiderivative of f increases exactly one unit over the interval [pic].
25.2.4 Because f is not continuous, there are other antiderivatives of f that pass through the point [pic]. Specifically, antiderivatives of f may or may not be continuous at [pic]. Draw onto figures 25.7 and 25.8 different antiderivatives of f that pass through the point [pic].
Problem 25.3
The function [pic] is an example of a periodic function. Specifically, the function has a period of [pic] because over any interval of length [pic] the behavior of the function is exactly the same as it was the previous interval of length [pic]. A little more precisely, [pic] regardless of the value of x.
Jasmine was thinking and told her lab assistant that derivatives and antiderivatives of periodic functions must also be periodic. Jasmine’s lab assistant told her that she was half right. Which half did Jasmine have correct? Also, draw a function that illustrates that the other half of Jasmine’s statement is not correct.
Problem 25.4
Hamid and Tasha were given the function g shown in Figure 25.9 and asked to identify where antiderivatives of the g are increasing. Hamid said that antiderivatives are increasing everywhere except for 2. Tasha replied that “antiderivatives of g are increasing everywhere, over the whole dang interval [pic].”
25.4.1 Who do you think is correct?
25.4.2 In the technical definition of increasing functions we say that a function f is increasing over the interval [pic] if for any two numbers on [pic], [pic] and [pic], it is always true that [pic] whenever [pic]. Does this definition support or contradict your conclusion in problem 25.4.1?
Activity 26
When given a function formula, we often find the first and second derivative formulas to determine behaviors of the given function. In a later lab we will use the first and second derivative formulas to help us graph a function given the formula for the function.
One thing we do with the derivative formulas is determine where they are positive, negative, zero, and undefined. This helps us determine where the given function is increasing, decreasing, concave up, concave down, and linear. It also helps us determine what happens when the function transitions from one state to the next.
Problem 26.1
As a warm-up exercise, several graphical situations are described below. Resolve each situation with work that supports your conclusion. In most instances, a simple picture will support your conclusion.
26.1.1 One day Sara and Jermaine were working on an assignment. One question asked them to draw a function over the domain [pic] with the properties that the function is always increasing and always concave down. Sara insisted that the curve must have a vertical asymptote at −2 and Jermaine insisted that the function must have a horizontal asymptote somewhere. Were either of these students correct?.
26.1.2 The next question Sara and Jermaine encountered described the same function with the added condition that the function is never positive. Sara and Jermaine made the same contentions about asymptotes. Is one of them now correct?
26.1.3 At another table Pedro and Yoshi were asked to draw a continuous curve that, among other properties, was never concave up. Pedro said ”OK, so the curve is always concave down” to which Yoshi replied “Pedro, you need to open your mind to other possibilities.” Who’s right?
26.1.4 In the next problem Pedro and Yoshi were asked to draw a function that is everywhere continuous and that is concave down at every value of x except 3. Yoshi declared “impossible” and Pedro responded “have some faith, Yosh-man.” Pedro then began to draw. Is it possible that Pedro came up with such a function?
Problem 26.2
26.2.1 Draw onto Figure 26.1 a continuous function f that has a horizontal tangent line at the point [pic]along with the properties stated in Table 26.1.
26.2.2 Given the conditions stated in problem 26.2.1, does it have to be the case that [pic]? Does it have to be the case that [pic]?
26.2.3 Is f increasing at [pic]? How do you know?
26.2.4 Can a continuous function satisfy the properties stated in Table 26.1 and not have a horizontal tangent line at [pic]? Draw a picture that supports your answer.
Problem 26.3
26.3.1 Draw onto Figure 26.2 a continuous function g that has a vertical tangent line at the point [pic]along with the properties stated in Table 26.2.
26.3.2 Given the conditions stated in problem 26.3.1, does it have to be the case that [pic] is undefined? Does it have to be the case that [pic] is undefined?
26.3.3 Is g increasing at [pic]? How do you know?
26.3.4 Can a continuous function satisfy the properties stated in Table 26.2 and not have a vertical tangent line at [pic]? Draw a picture that supports your answer.
Problem 26.4
26.4.1 Draw onto Figure 26.3 a continuous function k that passes through the point [pic] and also satisfies the properties stated in Table 26.3.
26.4.2 At what values of x is k nondifferentiable?
Problem 26.5
There are a total of 16 ways the empty boxes in Table 26.4 can be filled in with the words positive and negative (assuming one word per box). Determine all of the ways the boxes could be filled in that would make it impossible for f to be differentiable at [pic].
Problem 26.6
A function g is shown in Figure 26.4.
26.6.1 Let G be the continuous antiderivative of g that passes through the point [pic]. Draw G onto Figure 26.5.
26.6.2 At what values of t is G nondifferentiable?
26.6.3 At what values of t is g nondifferentiable?
Problem 26.7
Determine the correct answer to each of the following questions. Pictures of the situation may help you determine the correct answers.
26.7.1 Which of the following propositions is true? If a given proposition is not true, draw a graph that illustrates its untruth.
26.7.1a If the graph of [pic] has a vertical asymptote, then the graph of [pic] must also have a vertical asymptote.
26.7.1b If the graph of [pic] has a vertical asymptote, then the graph of [pic]must also have a vertical asymptote.
26.7.2 Suppose that the function f is everywhere continuous and concave down. Suppose further that [pic] and [pic]. Which of the following is true?
a. [pic] b. [pic] c. [pic]
d. There is not enough information to determine the relationship between [pic] and 11.
26.7.3 Suppose that the function f is everywhere continuous and increasing and that [pic]. Suppose further that F is an antiderivative of f that passes through the point [pic]. Which of the following is true?
a. [pic] b. [pic] c. [pic]
d. There is not enough information to determine the relationship between [pic] and 11.
-----------------------
Figure 26.3 k
Table 26.2: Signs on[pic] and [pic]
|Interval |[pic] |[pic] |
|[pic] |Positive |Positive |
|[pic] |Positive |Negative |
Figure 26.1 f
Table 26.1: Signs on[pic] and [pic]
|Interval |[pic] |[pic] |
|[pic] |Positive |Negative |
|[pic] |Positive |Positive |
Figure 26.2 g
Figure 25.9: g
Figure 25.8
Figure 25.7
Figure 25.6
Figure 25.5: f
Figure 25.4
Figure 24.6
Figure 24.7
Figure 24.5
Figure 22.7:[pic]
Figure 24.1
Figure 24.4
Figure 24.3
Figure 24.2
Table 24.3: [pic]and [pic]
|When [pic] is … |[pic] is … |
|Positive |Increasing |
|Negative |Decreasing |
|Constantly Zero |Constant |
|Increasing |Concave Up |
|Decreasing |Concave Down |
|Constant |Linear |
Table 24.2: [pic] and [pic]
|When [pic] is … |[pic] is … |
|Positive |Increasing |
|Negative |Decreasing |
|Constantly Zero |Constant |
|Increasing |Concave Up |
|Decreasing |Concave Down |
|Constant |Linear |
Table 24.1: [pic] and [pic]
|When [pic] is … |[pic] is … |
|Positive |Increasing |
|Negative |Decreasing |
|Constantly Zero |Constant |
|Increasing |Concave Up |
|Decreasing |Concave Down |
|Constant |Linear |
Figure 23.1:[pic]
Figure 23.2:[pic]
Figure 23.3:[pic]
Figure 23.4:[pic]
Figure 22.12:[pic]
Table 22.4: Choose the correct values for[pic]
|x |Proposed values |
|0 |[pic] or [pic] or [pic] |
|1 |[pic] or [pic] or [pic] |
|3 |[pic] or [pic] or 3 |
|5 |[pic] or [pic] or [pic] |
|6 |[pic] or [pic] or [pic] |
|8 |1 or 6 or 12 |
Table 22.2: Properties
|positive |
|negative |
|increasing |
|decreasing |
Table 22.3: Properties of [pic]
|Interval |Properties |
|[pic] | |
|[pic] | |
|[pic] | |
|[pic] | |
|[pic] | |
|[pic] | |
Figure 22.11: w
Figure 22.8:[pic]
[pic]
[pic]
Figure 22.6: k and [pic]
Figure 22.5: g and [pic]
[pic]
[pic]
Figure 22.4:[pic]
Figure 22.3: f
Table 22.1: [pic]
|x |y |
|−5 |6 |
|−3 | |
|−1 |2 |
|1 | |
|3 |−2 |
|5 |−4 |
|7 | |
Figure 22.2: g
Figure 22.1: g
Figure 21.1: f
Table 26.3: Signs on[pic] and [pic]
|Interval |[pic] |[pic] |
|[pic] |Negative |Positive |
|[pic] |Positive |Negative |
Table 26.4: Signs on[pic] and [pic]
|Interval |[pic] |[pic] |
|[pic] | | |
|[pic] | | |
Figure 26.5 G
Figure 26.4 g
[pic]
[pic]
Figure 22.9:[pic]
Figure 22.10:[pic]
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